Technical Library


The reactivity of vinyl-functional polymers is utilized in two major regimes. (Arkles, B., CHEMTECH, 1983, 13, 542.) Vinyl-terminated polymers are employed in addition-cure systems. The bond forming chemistry is the platinum catalyzed hydrosilylation reaction which proceeds according to the following equation:


Vinylmethylsiloxane copolymers and vinyl T-structure fluids are mostly employed in peroxide-activated cure systems, which  involve peroxide-induced free-radical coupling between vinyl and methyl groups. Concomitant and subsequent reactions take place among methyl groups and between crosslink sites and methyl groups. The initial crosslinking reaction is depicted in the following equation:

Mechanical Properties of Silicone Rubbers Formulated with Vinyl Silicones

Silicone TypeTensile Strength (MPa)Elongation (%)Tear Strength (kN/m)
HCR, high-consistency silicone rubber4-12100-1,1009-45
LSR, liquid silicone rubber4-12200-90010-50
RTV-2, room temperature vulcanizing silicone5-10100-7008-10
FSR, fluorosilicone rubber9-12150-70018-46

Addition-cure chemistry provides an extremely flexible basis for formulating silicone elastomers. An important feature of the cure system is that no byproducts are formed, allowing fabrication of parts with good dimensional stability. Cures below 50 °C—Room Temperature Vulcanizing (RTV), cures between 50 and 130 °C—Low Temperature Vulcanizing (LTV), and cures above 130 °C—High Temperature Vulcanizing (HTV) are all readily achieved by addition-cure. The rheology of the systems can also be varied widely, ranging from dip-cures to liquid injection molding (LIM) and conventional heat-cure rubber (HCR) processing. Vinyl-terminated polydimethylsiloxanes with viscosities > 200 cSt generally have < 2% volatiles and form the base polymers for these systems. More typically, base polymers range from 1,000 to 60,000 cSt. The crosslinking polymer is generally a methylhydrosiloxane-dimethylsiloxane copolymer with 15-50 mole % methylhydrosiloxane. The catalyst is usually a complex of platinum in alcohol, xylene, divinylsiloxanes, or cyclic vinylsiloxanes. The system is usually prepared in two parts. By convention, the A part typically has the vinyl-containing silicone and the platinum catalyst at a level of 5-10 ppm, and the B part usually contains the hydride-functional siloxane.

Formulation of addition cure silicones must address the following issues:

  • Strength – Unfilled silicones have extremely poor mechanical properties and crumble under pressure from a fingernail. The most effective reinforcing filler is hexamethyldisilazane-treated fumed silica. Alternatively, if clarity must be maintained, vinyl “Q” reinforcing resins are used.
  • Hardness – Higher crosslink density provides higher durometer elastomers. Gels are weakly crosslinked systems and even contain substantial quantities of “free” fluids. In principle, molar equivalents of hydrides react with vinyls. See the section on hydride-functional fluids for further information. Also, polymers with vinyl pendant on the chain rather than at chain ends are utilized to modify hardness and compression set.
  • Consistency – The viscosity of the base polymer and a variety of low surface area fillers ranging from calcium carbonate to precipitated silica are used to control the flow characteristics of silicone elastomers.
  • Temperature of cure – Selection of platinum catalysts generally controls the preferred temperature of cure. (J Inorg Organomet Polym 6, 123–144, 1996; J Mol Cat. A: Chem 104, 293, 1996) Platinum in vinyldisiloxanes is usually used in room temperature cures. Platinum in cyclic vinylsiloxanes is usually used in high temperature cures.
  • Work time (speed of cure) – Apart from temperature, moderators (sometimes called retarders) and inhibitors are used to control work time. Moderators slow but do not stop platinum catalysts. A typical moderator is tetravinyltetramethylcyclotetrasiloxane. Inhibitors stop or “shut-down” platinum catalysts and therefore are fugitive, i.e volatile or decomposed by heat or light (UV). Acetylenic alcohols such as methylisobutynol are volatile inhibitors. Patent literature shows that t-butylhydroperoxide is an effective inhibitor that breaks down at temperatures > 130 °C.
  • Low temperature properties/optical properties – The introduction of vinyl polymers with phenyl groups alters physical properties of elastomers. At levels of 3-4 mole %, phenyl groups improve low temperature properties. At higher levels, they are used to alter the refractive index of elastomers, ranging from matching fillers for transparency to optical fiber applications. Unfortunately, increased phenyl substitution lowers mechanical properties of elastomers.
  • Shelf-life – A fully compounded elastomer is a complex system. Shelf-life can be affected by moisture, differential adsorption of reactive components by fillers, and inhibitory effects of trace impurities. Empirical adjustments of catalyst and hydride levels are made to compensate for these effects.
  • Compounding – All but the lowest consistency elastomers are typically compounded in sigma-blade mixers, planetary mixers, two-roll mills or, for large scale production, twin-screw extruders.

Quick Start Formulation Transfer and Impression Molding Elastomer: This low strength formulation is useful as a reproductive molding compound. It is presented here because it can be prepared without special equipment and is an instructive starting point for addition cure silicone elastomers.

DMS-V31 [1000 cSt vinyl-terminated polydimethylsiloxane]: 100 parts
SIS6962.0 [hexamethyldisilazane-treated silica]: 50 parts
HMS-301 [methylhydrosiloxane-dimethylsiloxane copolymer]: 3-4 parts
SIP6830.3 [platinum complex solution]: 150-200 ppm
In small portions, work the DMS-V31 into the silica with a spatula. After a uniform dispersion is produced, work in the HMS-301. The blend may be stored in this form. Just prior to use, add the platinum solution with an eyedropper and work it in rapidly.  Working time is 5-10 minutes. The rate of cure can be retarded by adding tetravinyltetramethylcyclotetrasiloxane (SIT7900.0).

Activated-cure silicone elastomers are processed by methods consistent with conventional rubbers. These silicone products are referred to as HCRs (heat-cured rubbers). The base stocks are high molecular weight linear polydiorganosiloxanes that can be converted from a highly viscous plastic state into a predominantly elastic state by crosslinking. Vinylmethylsiloxane-dimethylsiloxane copolymers of extremely high molecular weights are the typical base stocks for activated-cure silicone elastomers. The base stocks are commonly referred to as gums. Gums typically have molecular weights from 500,000 to 900,000 with viscosities > 2,000,000 cSt. The silicone rubbers derived from the gums by compounding reinforcing agents extenders and additives are divided into three main classes: VMQ (dimethyl silicone/regular silicone), PVMQ (diphenyl dimethyl silicone/low temperature silicone) & FVMQ (fluorosilicone/fuel-resistant silicone).

Free-radical coupling (cure) of vinyl and methyl groups is usually initiated by peroxides at process temperatures of 140-160 °C. Generally, peroxide loading is 0.2-1.0%. Following the cure, a post-cure at 25-30 °C higher temperature removes volatile peroxide decomposition products and stabilizes polymer properties. The most widely used peroxides include dibenzoylperoxide (often as a 50% concentrate in silicone oil), dicumylperoxide, (often 40% on calcium carbonate), 2,5-dimethyl-2,5-di-t-butylperoxyhexane, and bis(dichlorobenzoyl)peroxide.1,2 The last peroxide is particularly recommended for aromatic-containing siloxanes. Terpolymer gums containing low levels of phenyl are used in low-temperature applications. At increased phenyl concentrations, they are used in high temperature and radiation-resistant applications and are typically compounded with stabilizing fillers such as iron oxide. Phenyl groups reduce crosslinking efficiency of peroxide systems and result in rubbers with lower elasticity. Fluorosilicone materials offer solvent resistance. Lower molecular weight vinylsiloxanes are frequently added to modify processability of base stocks.

1 Lynch, W., “Handbook of Silicone Rubber Fabrication”, Van Nostrand Reichold, 1978.
2 Brassard, D.M., “The Silicone Elastomer Handbook”, Silicone Solutions, 2010.

Peroxide and Peroxyketal Curing Agents for HTV Silicone Rubbers

PeroxideCure Temperature (°C)10 minute Half-Life Temperature (°C)Application
Dicumyl peroxide160-200157fast cure, calendering
Di(t-butylperoxy)diisopropylbenzene160-200157low odor
2,5-Dimethyl-2,5-di(t-butylperoxy)hexane160-200157commonly used for vinyl base stocks, FDA listed
2,5-Dimethyl-2,5-di(t-butylperoxy)hexyne190-200169systems requiring high temperature cure
2,4-Dichlorobenzoyl peroxide110-12589extrusion, phenyl copolymers, not vinyl specific
1,2-Bis(t-butylperoxy)3,3,5-trimethylcyclohexane135-185131fast-cure at lower temperatures without bloom
n-Butyl-4,4-di(t-butylperoxy)valerate135-185150high vinyl content cures

While the use of peroxide-activated cure chemistry for vinylmethylsiloxanes is well established for gum rubber stocks, its use is growing in new applications that are comparable to some peroxide-cure acrylic systems. Relatively low viscosity vinylmethylsiloxanes and vinyl T-fluids are employed as grafting additives to EPDM elastomers in the wire and cable industry to improve electrical properties. They also form reactive internal lubricants for vulcanizable rubber formulations. At low levels they are copolymerized with vinyl monomers to form surfactants for organosols.

Vinyl-Terminated Polydimethylsiloxanes, CAS: [68083-19-2] TSCA

Product CodeViscosity (cSt)Molecular WeightWt% VinylVinyl (eq/kg)Density
These materials are most often employed in 2-part addition cure silicone elastomers.

Monodisperse Vinyl-Terminated Polydimethylsiloxanes

Product CodeViscosity (cSt)Molecular WeightWt% VinylVinyl (eq/kg)Density
Monodisperse telechelic silicone fluids offer advantages over traditional telechelic fluids. These materials contain little or no low molecular weight non-functional components which can plasticize and migrate out of cured elastomers, reducing or eliminating migratory contamination issues.

Reduced Volatility*, Vinyl-Terminated Polydimethylsiloxanes

Product CodeViscosity (cSt)Molecular WeightWt% VinylVinyl (eq/kg)Density
*total volatiles, 4 hours @ 150 °C: 0.2% maximum

Fumed Silica Reinforced Vinyl-Terminated Polydimethylsiloxanes

Product CodeViscosity (cSt)Base Fluid Viscosity (cSt)Wt% SilicaVinyl (eq/kg)Density
*total volatiles, 4 hours @ 150 °C: 0.2% maximum

Precompounded base materials provide access to low durometer formulations without the need for special compounding equipment required to mix fumed silica. The following is a starting-point formulation.

Part A:

Part B:

Prepare Part A and Part B separately. When ready to cure, mix 3 parts A to 1 part B. The mix will cure over 4 hours at room temperature to give the following properties:

  • Hardness: 20-30 Shore A
  • Tensile Strength: 3.5 MPa (500 psi)
  • Elongation: 400-450%
  • Tear Strength: 16 N/mm (91 ppi)

Vinyl-Terminated Diphenylsiloxane-Dimethylsiloxane Copolymers, CAS: [68951-96-2] TSCA

Product CodeMole % DiphenylsiloxaneViscosity (cSt)Molecular WeightVinyl (eq/kg)Refractive Index

Vinyl-Terminated Polyphenylmethylsiloxane, CAS: [225927-21-9], TSCA-L

Product CodeMole % PhenylmethylsiloxaneViscosity (cSt)Molecular WeightVinyl (eq/kg)Refractive IndexDensity
These materials are most often employed in 2-part addition cure silicone elastomers where special thermal or optical
properties are required.

Vinylphenylmethyl-Terminated Vinylphenylsiloxane-Phenylmethylsiloxane Copolymer, CAS: [68037-82-1], TSCA

Product CodeMole % PhenylmethylsiloxaneViscosity (cSt)Molecular WeightVinyl (eq/kg)Refractive IndexDensity
Crosslinks with dicumyl peroxide.

Vinyl-Terminated Trifluoropropylmethylsiloxane-Dimethylsiloxane Copolymers, CAS: [68951-98-4], TSCA

Product CodeMole % TrifluoropropylViscosity (cSt)Molecular WeightRefractive IndexDensity
Trifluoropropylmethylsiloxane copolymers offer greater solvent resistance (lower hydrocarbon solubility) and lower
refractive index than analogous dimethylsiloxane homopolymers.

Vinyl-Terminated Nonafluorohexylmethylsiloxane-Dimethylsiloxane Copolymer, CAS:[609768-44-7]

Product CodeMole % NonalfuorohexylViscosity (cSt)Molecular WeightRefractive IndexDensity

Vinyl-Terminated Diethylsiloxane-Dimethylsiloxane Copolymer

Product CodeMole % DiethylsiloxaneViscosity (cSt)Molecular WeightRefractive IndexDensity
Diethylsiloxane copolymers offer better hydrocarbon compatibility (greater solubility) and higher refractive index than analogous dimethylsiloxane homopolymers.

Vinyl-Terminated Ethylene-Siloxane Copolymer Fluids, CAS: [26710-23-6]

Product CodeMole % SiloxaneViscosity (cSt)Molecular WeightRefractive IndexDensity
Ethylene-siloxane copolymer polymers exhibit reversion resistant behavior.

(Also see MCS-V212)

Vinylmethylsiloxane-Dimethylsiloxane Copolymers, Trimethylsiloxy-Terminated, CAS: [67762-94-1], TSCA

Product CodeMole % VinylmethylsiloxaneViscosity (cSt)Molecular WeightVinyl (eq/kg)Density

Vinyl-containing copolymers are used as crosslinkers in Pt- and peroxide-cure elastomer. High vinyl content copolymers form elastomers used in high accuracy soft lithography.1,2,3

1. Choi, D. et al. Mat. Sci. Eng. C. 2004, 24, 213.
2. Infuehr, R. et al. Appl. Surf. Sci. 2003, 254, 836.
3. Schmid, H.; Michel. B. Macromolecules 2000, 33, 3042.

Vinylmethylsiloxane-Dimethylsiloxane Copolymers, Silanol-Terminated, 4-6% OH, CAS: [67923-19-7], TSCA

Product CodeMole % VinylmethylsiloxaneViscosity (cSt)Molecular WeightVinyl (eq/kg)Density

Vinylmethylsiloxane-Dimethylsiloxane Copolymers, Vinyl-Terminated, CAS: [68083-18-1] TSCA

Product CodeMole % VinylmethylsiloxaneViscosity (cSt)Molecular WeightVinyl (eq/kg)Density
These materials are modifiers for addition cure and activated cure elastomers.

Vinylmethylsiloxane-Dimethylsiloxane Copolymers, Hydride-Terminated

Product CodeViscosity (cSt)Molecular WeightVinyl (eq/kg)Refractive IndexDensity

Vinyl Gums (balance dimethylsiloxane unless otherwise specified), TSCA

Product CodeMole % VinylmethylsiloxaneComonomer %DensityCAS
VGP-0610.1-0.26-7% diphenylsiloxane0.99[68951-96-2]
VGF-9911.0-2.098-99% trifluoropropylmethylsiloxane1.35[68952-02-2]
DGM-000*0.0100% dimethylsiloxane0.98[9016-00-6]
These materials are base polymers for activated cure specialty silicone rubbers.
*This gum is listed here for convenience. It contains no vinyl functionality. It may be cured with dichlorobenzoylperoxide.

Vinyl Q Resin Dispersions, CAS: [68584-83-8], TSCA

Product CodeBaseViscosity (cSt)Vinyl (eq/kg)Refractive IndexDensityComments
VQM-135DMS-V414,500-70000.2-0.31.4051.0220-25% Q-resin
VQM-146DMS-V4650,000-60,0000.18-0.231.4061.0220-25% Q-resin
VQX-22150% in xylene--0.4-0.6--1.05
Vinyl Q resins are clear reinforcing additives for addition cure elastomers.

Vinylmethylsiloxane Homopolymers, TSCA

Product CodeDescriptionViscosity (cSt)Molecular WeightDensity
VMS-T11linear (CAS: [68037-87-6])7-151,000-1,5000.96
Low molecular weight vinylmethylsiloxanes are primarily used as moderators (cure-rate retarders) for vinyl-addition
cure silicones. They also are reactive intermediates and monomers.

See also Hydride Q resins.

Vinyl T-structure Polymers

Product CodeBranch PointBranch TerminusVinyl (eq/kg)Viscosity (cSt)Refractive IndexDensityCAS
VTT-106vinylmethyl2-45-8--0.90[126581-51-9], TSCA
MTV-112methylvinyl3-615-301.4070.96CAS: [21714-00-0]
T-structure polymers contain multiple branch points.
These materials are additives and modifiers for addition-cure and activated-cure elastomers.

Hetero bi-functional silicone fluids contain little or no low molecular weight components. They can be used as additives into traditional RTV-2 silicone formulations or undergo a stepgrowth process when catalyzed by platinum, resulting in high elongation elastomer.1,2

1. Goff, J. et al, Polymer Preprints 2012, 53(1), 487.
2. Goff, J. et al, Advanced Materials, 2016, 28(12), 2393, doi 10.1002/adma, 201503320

α-Monovinyl-Ω-Monohydride-Terminated Polydimethylsiloxane, CAS: [104780-63-4], TSCA

Product CodeViscosity (cSt)Molecular WeightRefractive IndexDensity

α-Monovinyl-Monophenyl-Ω-Monohydride-Terminated Polydimethylsiloxane, CAS: [1422279-25-1]

Product CodeViscosity (cSt)Molecular WeightRefractive IndexDensity
Monovinyl-functional silicone fluids can be used as components in silicone gels and modifiers in release coatings.

Monovinyl-Terminated Polydimethylsiloxane - asymmetric, CAS: [68951-99-5]

Product CodeViscosity (cSt)Molecular WeightRefractive IndexDensity

Monovinyl-Functional Polydimethylsiloxane - symmetric, CAS: [689252-00-1]

Product CodeViscosity (cSt)Molecular WeightRefractive IndexDensity

(3-5% Vinylmethylsiloxane)-(35-40% R)-(Dimethylsiloxane) Terpolymer

Product CodeRViscosity (cSt)Molecular WeightVinyl (eq/kg)Refractive IndexDensityComments
VAT-4326*octylmethylsiloxane500-70010,000-12,0000.20-0.241.4370.93Vinyl-alkyl terpolymers are used in hybrid organic polymer-silicone applications.
Employed as a matrix polymer in vapor sensor films. (Blok, E. et al, US Patent 7,138,090, 2006.)
VPT-1323phenylmethylsiloxane250-3502,500-3,0000.25-0.291.4671.03Vinyl-phenyl terpolymers are used in refractive index match applications.
*CAS: [597543-32-3], TSCA

Dimethylsiloxane-Vinylmethylsiloxane - (Propylene Oxide-Ethylene Oxide) Block Copolymers

Product CodeViscosity (cSt)Molecular WeightVinyl (eq/kg)Refractive IndexDensity
Vinyl functional glycol-silicone copolymers are used as hydrophilic additives in silicone RTV-2 formulations.

Polydimethylsiloxane, Bis(divinyl)-Terminated

Product CodeViscosity (cSt)Molecular WeightVinyl (eq/kg)Refractive IndexDensity

Vinylalkoxysiloxane Homopolymer - oligomers, TSCA

Product CodeAlkoxyViscosity (cSt)Wt% VinylRefractive IndexDensityCAS

Vinylethoxysiloxane-Propylethoxysiloxane Copolymer - oligomers, CAS: [201615-10-3], TSCA

Product CodeViscosity (cSt)Wt% VinylRefractive IndexDensity
These materials are employed as adhesion promoters for vinyl-addition cure RTVs, as crosslinking agents for neutral cure RTVs, and as coupling agents in polyethylene for wire and cable applications.